KNEE-JOINT. The articulation between the femur or thigh-bone, above. and the tibia or shin bone below. A third hone. the patella or knee cap—one of the sesamoid bones (q.v.), and not a true bone of the skeleton—also enters into the structure of this joint anteriorly. The articular surfaces of these hones are covered with cartilage, lined by a synovial membrane. which is the most extensive in the body. and connected together by ligaments, some of which lie external to the joint, while others occupy its interior.
The most important of the external ligaments are the anterior of ligamention patella•, which is in reality that portion of the quadriceps extensor cruris which is continued from the kneecap to the tubercle of the tibia; one internal, and t•vc external lateral ligaments; a posterior ligament ; and a capsular ligament, which surrounds the joint in the intervals left by the preceding liga ments. The positions of these ligaments are suf ficientl• indicated by their names. Of the in ternal ligaments, the two crucial, so called because they cross each other, are the most im iortant The external and internal semi-lunar 1 • • cartilages are two erescentrie plates of cartilage: the outer part of each is thick, the inner free border thin. Each cartilage covers nearly the
outer two-thirds of the corresponding articular surface of the tibia, and by its form deepens these surfaces for firmer articulation with the COD • dyle: of the femur.
The chief movement, of this joint are those of a hinge-joint—namely. flexion and extension; but it is also capable of slight rotatory motion when the knee is half-flexed. During flexion the articular surfaces of the tibia glide backward upon the condyles of the femur: while in exten sion they glide forward. The whole range of motion of this joint. from extreme flexion to extreme extension, is about 150 degrees. Judging from its articular surfaces. which have compara tively little adaptation for each other, it might he inferred that this was a weak and insecure joint ; and yet it is very rarely dislocated.